Radar systems for motor-vehicle applications, in particular driver-assistance systems such as ACC (Automatic Cruise Control) systems, which are based on the radar principle, may require a directionally resolved image of the vehicle surroundings (distance information/angle information) for evaluation. In determining the angle, current systems may operate according to monopulse methods, which are based on a comparison of amplitudes and phases of several reception directivity characteristics. These methods may achieve a sufficient resolution in a relatively narrow visual range, as long as only one target is situated in a distance/velocity cell. If a plurality of targets is situated in one cell, incorrect angles may result.
A multibeam antenna system having a plurality of radiator elements, which are coupled to a beam-shaping network that takes the form of a Butler matrix is discussed in PCT application number WO 02/15334 A1. The Butler matrix is connected to a switchable beam-combination network. Used there for the transmitting and receiving directions are antenna pairs, whose radiation patterns can be simultaneously controlled, in order to ensure that they point in the same direction.
A set-up for calibrating a transmitter and/or receiver for controlling a beam-forming network is referred to in German Published Patent Application No. 196 44 686. This allows the same beam characteristic to be set for transmit and receive modes, without the highly precise transmitter or receiver being necessary. An automatic calibration system for radar applications is referred to in U.S. Pat. No. 5,682,165. A radar sensor having a Rotman lens and group antenna is referred to in PCT application number WO 01/09975 A2. A control circuit takes the form of a transmitting and receiving module. Separate sensitivities can be set for different angular segments, in order to detect objects in an angularly resolved manner. In addition, distances and speeds, particularly with respect to preceding vehicles, can be determined. The evaluation is performed digitally.